The present invention relates to a DC circuit breaker and a method of commutation thereof, or more in particular to a system for inserting a reverse current by use of a commutation capacitor capable of direct charge from a DC line.
A basic circuit of a conventional DC circuit breaker of reverse current insertion type is shown in FIG. 1. This type of DC circuit breaker is described in JP-A-No. 54-149873.
As shown in FIG. 1, a conventional DC circuit breaker of reverse current insertion type is configured of a breaker unit 2 inserted in series in a main DC line 1, and a series connection of a commutation capacitor 3, a reactor 4 and a first switch 5 including a thyristor or a trigger gap inserted in parallel to the breaker unit 2. In FIG. 1, reference numeral 1a designates a power-side terminal of the DC line 1, numeral 1b a load-side terminal of the DC line 1, numeral 6 a charge unit connected in parallel to the commutation capacitor 3, numeral 7 a negative bus, numeral 7a a power-side terminal of the negative bus 7, and numeral 7b a load-side terminal of the negative bus 7. Further, a load 8 is connected between the load-side terminals 1b and 7b. A DC power supply V is inserted between the power-side terminals 1a and 7a.
In FIG. 1, in a circuit-breaking operation, an open command is applied to the breaker unit 2, and after the breaker unit 2 is opened, the thyristor switch 5 making up the first switch is turned on, that is, fired, so that the discharge current .circle.2 from the commutation capacitor 3 charged in advance from the charge unit 6 is supplied in the direction reverse to the energization current .circle.1 of the breaker unit 2. The sum of the currents .circle.1 and .circle.2 flowing through the breaker unit 2 develops a point of zero, with the result that the arc of the breaker unit 2 is extinguished, and the line current .circle.1 commutates to the commutation capacitor 3, thereby completing the current-limiting process.
The above-mentioned conventional system, however, requires a charge unit 6 exclusively used for charging the commutation capacitor 3. In the case where the capacitor is charged directly from the DC line 1, on the other hand, the charging from the load side of the breaker unit 2 is necessary depending on the charging polarity, and the breaking function is required to be given up until the capacitor 3 is completely charged after the breaker unit 2 is turned on, thereby hampering practical applications of this sytem. In the case where an exclusive charge unit 6 is inserted, it is also necessary to insert an inverter for introducing power from a DC battery or the DC line 1 to assure power reliability. This complicates the system on the one hand and reduces the reliability of the DC line at the same time.